Control of motion and force is one of the basic elements in mechanical engineering. Development of new materials has made it possible to produce motion and force using special functional materials called actuator materials. The most important groups of actuator materials available are piezoelectric ceramics, magnetostrictive intermetallics, and shape memory alloys. Piezoelectric ceramics develop strains when subjected to an electric field. Frequency response of these materials is fast, but the strain amplitudes are very small, which limits their applicability. Magnetostrictive materials are strained when a magnetic field is imposed on them. Certain high-magnetostrictive intermetallics (e.g., Terfenol-D, Etrema Products, Inc., Ames, IA, USA) offer strains up to 0.17%, which is an order of magnitude higher than those of the current piezolectrics. The frequency response of the magnetostrictive intermetallics is lower than that of the piezoelectrics.
Shape memory metals are materials which, when plastically deformed at one temperature, can recover their original undeformed state upon raising their temperature above an alloy-specific transformation temperature. In these materials, crystal structure undergoes a phase transformation into, and out of, a martensite phase when subjected to mechanical loads or temperature. The process when a mechanically deformed shape memory material returns to its original form after heating is called a one-way shape memory effect. Cooling the material subsequently will not reverse the shape change. The one-way shape memory effect is utilized in fastening, tightening and prestressing devices. Strains of several percent can be completely recovered, and recovery stresses of over 900 MPa have been attained. In the case of the two-way effect, no deformation is required, and the material "remembers" two configurations that are obtained by heating and cooling to alloy-specific temperatures. The temperature difference between the two configurations can be as small as 1 to 2 K. Materials that exhibit a two-way shape memory effect are used to develop forces and displacements in actuators. Those actuators are applied in machinery, robotics and biomedical engineering. The most extensively used shape memory materials are Ni--Ti and Cu-based alloys. A drawback of the shape memory actuators is their slow response due to the thermal control (especially in cooling) and low efficiency (energy conversion), which in many alloys is only about one percent.
In order for the shape memory effect to occur, the material must exhibit a twinned substructure. The shape change of the shape memory material is based on the reorientation of the twin structure in the external stress field. A two-dimensional illustration of the twin reorientation is presented in FIG. 1. FIG. 1(a) shows two twin variants, denoted by 1 and 2, with equal proportions in the absence of the external stress. When the stress is applied, FIG. 1(b), the twin boundaries move and variant 2 grows at the expense of variant 1, producing the shape which better accommodates the applied stress. The result of moving a twin boundary is thus to convert one twin variant into another. The variants which are most favorably oriented to the applied stress will grow. Ultimately, a single variant of martensite can be produced by straining a sufficient amount, as illustrated in FIG. 1(c). In the martensite phase, the variants are usually oriented in several crystallographic directions. Therefore, complex shape changes of the material can be produced by the reorientation of the twin structure, and a full shape recovery will be obtained. Crystallographic analysis has shown that the boundaries between the martensite plates also behave as twin boundaries, i.e., the individual plates of martensite themselves are twins with respect to adjoining plates. Thus the term "twin boundaries", generally refers to the boundaries between martensite plates as well as the boundaries between the boundaries within the plates (this definition also concerns the magnetically controlled twin boundaries discussed below). In some materials, applied stress induces formation of the martensite phase whose twinned substructure is preferentially oriented according to the applied stress.
Reorientation of the twin structure is responsible for the recoverable strains of several percent in appropriate materials (e.g., close to 10 percent in Ni--Ti shape memory alloys). In some alloys the stress required to reorient the twin structure is very low. FIG. 2 shows the stress-strain curves for the selected shape memory materials. It is seen that strains of 4 percent are attained by stresses of 20 to 50 MPa in most of those alloys. Stresses as low as 1 to 30 MPa result in strains of one percent. Strain energy densities needed to produce the strain of 1 percent by the reorientation of the twin variants are the areas restricted by the stress-strain curves, strain axis and the vertical dashed line in FIG. 2. The strain energy densities for the alloys In--TI, Ni--Mn--Ga (ferromagnetic Ni.sub.2 MnGa), CuZn--Sn and Cu--Zn are 10.sup.4, 8.5.times.10.sup.4, 1.1.times.10.sup.5 and 2.3.times.10.sup.5 J/m.sup.3, respectively.
In the following, magnetic anisotropy energy is introduced, because it plays an important role in the present invention. In ferromagnetic crystals magnetocrystalline anisotropy energy is an energy which directs the magnetization along certain definite crystallographic axes called directions of easy magnetization. FIG. 3 shows the magnetization curves of single crystalline cobalt which has a hexagonal crystal structure. Its easy direction of magnetization is the parallel axis of the unit cell. The saturation is reached at a low magnetic field value in this direction, as shown in FIG. 3. Saturating the sample in the basal plane is much more difficult. A magnetic field over 8000 Oe is needed for saturation. The basal plane direction is called a hard direction of magnetization. Magnetic anisotropy energy density corresponding to the magnetization processes in different directions is the area between the magnetization curves for those directions. In cobalt the energy density needed to saturate the sample in the hard direction is about 5.times.10.sup.5 J/m.sup.3 (the area between the saturation curves in FIG. 3). Anisotropy energy densities of magnetically hard Fe- and Co-based alloys range from 10.sup.5 up to 10.sup.7 J/m.sup.3. The highest anisotropy energy densities (K1 values), close to 10.sup.8 J/m.sup.3, are in 4f metals at low temperatures. In intermetallic compounds such as Co.sub.5 Nd, Fe.sub.14 Nd.sub.2 B and Sm.sub.2 Co.sub.17 the anisotropy energy densities at room temperature are 1.5.times.10.sup.7, 5.times.10.sup.7 and 3.2.times.10.sup.6 J/m.sup.3, respectively.